This invention relates to circuits for energizing gas discharge lamps, and more specifically relates to a novel control circuit for a gas discharge lamp which can permit the dimming of lamps associated with conventional non-dimming ballasts.
Gas discharge lamps are widely used as illumination sources. As hereinafter used, gas discharge lamps include fluorescent lamps with or without separate heaters. High Intensity Discharge (HID) lamps, and all other lamps which generally exhibit a negative resistance characteristic. Such lamps require ballast circuits to provide a stable operating condition when they are used with standard a-c power sources. This is because the plasma arc within the lamp has a negative resistance characteristic which requires a series ballast impedance to achieve a stable operating point. Other functions of ballasts are to provide additional striking voltage to start the lamp initially and, in some cases, to provide power for internal lamp cathode heaters.
The ballasts are usually installed in or very near each lighting fixture containing the one or more lamps with which they are associated. Generally each ballast will only operate one or two lamps. The ballast is mounted in close proximity to its lamps and generally directly in the same fixture to make it self-contained and simplify wiring during assembly. Consequently, access to the interior circuitry of a ballasted gas discharge lamp assembly is physically limited. Moreover, the fixture is frequently mounted overhead so that access to the fixture and the ballast components is limited.
It is known to be desirable to modify existing non-dimming gas discharge lamp assemblies so that their output light can be modified or dimmed when 100% of their available light output is not necessary. It would also be desirable to make new lamp installations with the dimming capability but using commercially available and relatively inexpensive non-dimming ballasts.
Thus, substantial energy savings can be made if the output of gas discharge lamps is reduced when regions they are to illuminate are partly illuminated by other sources such as sunshine entering a room to be illuminated. Energy can also be saved by reducing the output of a gas discharge lamp when it is new and when its output is substantially greater than at the end of its useful life. Known systems provide lamp dimming which will provide a given ambient illumination so that the energy used by the lamp is only the energy needed to bring the illumination level in a given area at a given time to its desired value. This can substantially reduce energy cost and use.
Existing systems can be modified to be capable of dimming by replacing the existing ballast in a fixture with a ballast capable of operation in a dimming mode, or by suitably controlling the input power. Thus, the gas discharge lamp ballast can be replaced by a variable series inductor. This, however, is an expensive and complex structure and, moreover, the device could not be retrofitted easily into a standard fixture.
It is also possible to provide a variable amplitude a-c input source through the use, for example, of an autotransformer while maintaining a fixed ballast impedance. The variable autotransformer, however, is expensive and physically large. Moreover, the line voltage in such a device would have to be substantially higher than lamp operating voltage to permit striking of the lamps. Furthermore, means must be provided to prevent the reduction of heater voltage if the lamps employ a cathode heater since the operation of the lamps at low heater voltage will substantially reduce their life.
Other arrangements have been proposed employing series ballast inductances which can be selectively short-circuited as shown, for example, in U.S. Pat. No. 3,816,794. A device of this type is not well suited for retrofit installation and is very costly since its use would require the dismantling of existing fixtures and the running of additional conductors to enable the selective short-circuiting of one or more of the inductors.
Dimming ballasts are also known which use thyristor type circuits for controlling the application of a phase controlled input current to a gas discharge lamp, such as a rapid-start fluorescent lamp. In these arrangements, the primary winding of the dimming ballast is always at full line voltage so that heater voltage can be kept high during the dimming cycle. However, it would be very difficult to modify an existing gas discharge lamp installation to employ such a dimming ballast since it would require access to and modification of the ballast in the fixture and additional wiring to the fixtures.
The need for an additional wire for a dimming ballast can be eliminated by using a ballast circuit of the type shown in U.S. Pat. No. 3,422,309, entitled FLUORESCENT LIGHT DIMMING SYSTEM, in the name of Spira et al, and assigned to the assignee of the present invention. In this device, thyristors are disposed in series with a two-wire dimming ballast. Special circuits are needed to maintain heater voltage at a sufficiently high level during dimming to prevent damage to the tube. Moreover, the retrofitting of this ballast into an existing non-dimming ballast installation would be complex and expensive.
The ballast configuration of U.S. Pat. No. 3,422,309 above uses conventional phase control, whereby the firing angle of the thyristor is delayed by a greater or lesser extent to control the conduction time during which current is applied to the ballast. Other control systems are known which employ a form of reverse phase control, whereby current flow begins at the beginning of a half cycle but is terminated before the end of the half cycle. By terminating the point at which current flow is stopped, one employs a form of phase control. Circuits of this type have been manufactured and sold under the name Ecostat by Evers GmbH; Eichofstrasse 14, 2300 Kiel 1 W. Germany.
The Ecostat arrangement permits energy stored in a reactor ballast and power factor correction capacitor to be discharged into the gas discharge lamp after a transistorized a-c switch is opened. This then serves to limit the deionization of the gas discharge lamp during the switch-off interval. The use of this arrangement in an existing installation, would, however, require the complex modification of the standard non-dimming ballast.
The use of reverse phase-controlled circuits for dimming incandescent lamps is also disclosed in a paper by Burkhart and Ostrodaki, entitled REVERSE PHASE-CONTROLLED DIMMER FOR INCANDESCENT LIGHTING, in the I.E.E.E. Transactions on Industrial Applications, Volume 1A-15, No. 5, September/October 1979, pages 579 through 583.
Another method for ballasting of gas discharge lamps, so they can be dimmed, is the use of an electronic current limiting circuit in place of the standard magnetic ballast as is shown and described in U.S. Pat. No. 3,619,716, entitled HIGH FREQUENCY FLUORESCENT TUBE LIGHTING CIRCUIT AND A-C DRIVING CIRCUIT THEREFOR, Spira et al, and assigned to the assignee of the present invention. While this device achieves increases in efficacy of up to 25% with fluorescent lamps and somewhat less for high intensity discharge lamps and produces very attractive performance, the system would also require major modification of a non-dimming installation to be retrofitted.
Another dimming arrangement is known, made by Controlled Environment Systems Inc., of Rockville, Md., known as the "E.C.A.L.O." system. This system operates a fluorescent lighting system having standard ballasts in a dimming mode.
Most installations containing non-dimming ballasts will contain a ballast design which is a type known as a "regulating autotransformer ballast". The so-called regulating autotransformer ballast consists of an autotransformer having a primary winding portion connected to the a-c mains and a secondary winding portion connected in closed series relation with a series capacitor and the gas discharge lamp or lamps. The primary and secondary portions are loosely coupled by the autotransformer leakage inductance.
None of the known gas discharge lamp control systems described previously provide satisfactory performance when used with regulating autotransformer ballasts. Thus, the use of a series impedance or autotransformer scheme results in rapid loss of filament voltage and cycle-to-cycle restriking voltage, resulting in limited control range before the lamps either go out or are in danger of damage due to low heater voltage.
Conventional phase control schemes and the reverse phase angle control schemes, when applied to the conventional regulating autotransformer ballast, will provide significant dimming control to about 40% or less of the rated output before the lamps go out. However, line power factor deteriorates very rapidly so that the RMS line current into the system might actually increase as the lamp output is reduced. This increase can be as much as 50% above the line current at 100% rated light output when lamp output is reduced to about 30% with a high intensity discharge lamp. This would then increase ballast and distribution system losses and increase line current to the extent it might cause branch circuit breakers to operate. Also the amount of ballast input voltage reduction required to obtain satisfactory dimming will result in lamp filament voltages of rapid start fluorescent lamps being reduced to such an extent as to have an adverse effect on lamp life and dimming control.